1. Field of the Invention
The present invention relates to a document reading device, an image forming apparatus, a document reading method, and a computer-readable storage medium.
2. Description of the Related Art
Conventionally, there has been known an image reading device that includes a box-shaped housing having an opened top surface, a flat contact glass which is attached to the opened top surface of the housing, and a reading unit such as a contact image sensor (hereinbelow, referred to as a CIS) which is reciprocatably provided under the contact glass. In such a flatbed image reading device, a document is placed on the surface of the contact glass so that a surface to be read of the document faces downward, an image on the document is optically read by the CIS, an optical signal of the read image is converted into an electrical signal, and image data is thereby generated.
Recently, there has also been actively developed a sheet through reading device that reads document image data by conveying a document while an image reading unit remains in a stopped state. A sheet through reading device is generally mounted on the above-described flatbed image reading device. When performing flatbed document reading, the sheet through reading device often also serves as a document pressing unit. When an image reading device has a sheet through reading function, a contact glass for sheet through reading is installed in addition to the contact glass for loading a document thereon.
In a general image reading device as described above, a reference stop position of the image reading unit (a CIS, for example) in a standby state is often located under a sheet through reading contact glass. The reason thereof is that, in order to reduce moving operations of the image reading unit as far as possible when document reading is performed by a sheet through method, it is the best way to set the stop position so as to coincide with a sheet through reading position.
When using an image reading device, some users often perform flatbed document reading while a document pressing member (a pressurizing plate or a feeder for sheet thorough reading) is in an open state (for example, when copying a thick literature). In the image reading device having such a configuration, when the document pressing unit is in an open state, ambient light disadvantageously enters the inside of the device through the surface of the contact glass. Ambient light is indoor or outdoor light that enters the inside of the device through an contact glass or an opening of the device.
As described above, in the image reading device in which the reference stop position of the image reading unit coincides with the sheet through reading position, when the document pressing member is in an open state, ambient light directly affects the device.
As a technique for reducing the influence of ambient light, for example, Japanese Patent No. 4457976 discloses a configuration in which a black reference member is disposed to be adjacent to a white reference member for the purpose of reducing variations in black correction data and white correction data caused by ambient light to reduce the influence of ambient light, thereby preventing the deterioration in image quality. However, in Japanese Patent No. 4457976, there are adverse effects such as unavoidable cost increase in the entire device due to the addition of the black reference member and the increase in size of the device. Further, after obtaining black correction data and white correction data at the position near the end of a document, an operation is immediately shifted to a document read operation. However, in this case, it is necessary to control the reading speed with high acceleration so as to reach a predetermined reading speed. In this case, the speed of a reading unit is unstable in the leading end area of the document, and the deterioration in image quality may occur due to uneven speed.
When an image reading unit that uses an image sensor without an optical light-shielding pixel (optical black) is used, generally, upon receiving an instruction to start a document read operation, correction data for black level correction (black shading) is generated and held on the basis of a signal output from the sensor while a light source is in a turned-off state, the black correction data is then subtracted from an output signal obtained from reflected light from a document or a reference white plate, and various corrections are thereafter performed. Normally, upon receiving an instruction for the document read operation, it is preferred to perform control in such a manner that black correction data is first generated and held at the reference stop position while the light source is in a turned-off state, and the image reading unit is then moved to shift an operation to the document reading.
However, in the above conventional technique, when the black correction data generation as described above is performed while the document pressing unit is in an open state, an output signal having a larger output level than that of an actual dark time output signal is disadvantageously obtained due to ambient light as shown in the graph of FIG. 13 illustrating an output level (vertical axis) with respect to a pixel position (horizontal axis). Further, since the way of entrance of ambient light with respect to the image sensor variously changes depending on the installation environment of the image reading device, the amount of incident light is highly likely to change depending on the main scanning position. Normally, when a document having even density in the main scanning direction is read, image data having even density should be output. However, in the above case, different outputs are obtained depending on the main-scanning position. As a result, it is not possible to prevent uneven density and the generation of lines in a final image, and a significant deterioration in image quality thereby disadvantageously occurs.
Therefore, there is a need to prevent the deterioration in image quality even when a document pressing member is in an open state by obtaining dark time output data while the influence of ambient light is minimized.